- TILT SHIFT LENSES: DEPTH OF FIELD -

Tilt shift lenses enable photographers to transcend the normal restrictions
of depth of field and perspective. Many of the optical tricks these lenses permit
could not otherwise be reproduced digitally—making them a must for certain landscape,
architectural and product photography. This part of the tutorial addresses the
tilt feature, and focuses on its use in digital SLR cameras for controlling
depth of field. The first part of this tutorial focused on
using tilt shift lenses to control perspective and create panoramas.

OVERVIEW: TILT SHIFT MOVEMENTS

Shift movements enable the photographer to shift the location of the
lens's imaging circle relative to the digital camera sensor. This means that
the lens's center of perspective no longer corresponds the the image's center
of perspective, and produces an effect similar to only using a crop from the
side of a correspondingly wider angle lens.

Tilt movements enable the photographer to tilt the plane of sharpest
focus so that it no longer lies perpendicular to the lens axis. This produces
a wedge-shaped depth of field whose width increases further from the camera.
The tilt effect therefore does not necessarily increase depth of field—it just
allows the photographer to customize its location to better suit their subject
matter.

CONCEPT: SCHEIMPFLUG PRINCIPLE & HINGE RULE

The Scheimpflug principle states that the sensor plane, lens plane and plane
of sharpest focus must all intersect along a line. In the diagram below, this
intersection is actually a point since the line is perpendicular to your the
screen. When the Scheimpflug principle is combined with the "Hinge" or "Pivot
Rule," these collectively define the location for the plane of sharpest focus
as follows:

Try experimenting with different values of tilt to get a feel for how this
influences the plane of sharpest focus. Notice that even a small lens tilt angle
can produce a correspondingly large tilt in the plane of sharpest focus.

The focusing distance can also change the plane of sharpest focus along with
tilt, and will be discussed later in this tutorial. Also note that for the sake
of brevity, the rest of this tutorial will use "plane of sharpest focus" and
"focus plane" synonymously.

TILT MOVEMENTS TO REPOSITION DEPTH OF FIELD

Depth of field for many scenes is often insufficient using standard equipment—even
with small lens apertures. The problem is that one could use even smaller apertures
to increase depth of field, but not without also increasing
softness at the camera's focus plane due to diffraction. Tilt movements
can sometimes avoid this technical limitation by making more efficient use of
the depth of field, depending on the subject matter.

The example below demonstrates the effect of tilt movements on a scene whose
subject matter traverses both up/down and front/back directions. Each image
is taken using a wide aperture of f/2.8 to make the depth of field more noticeable
at this small image size.

On the left we see the typical depth if field produced by an ordinary lens.
In order to get both the front and rear rug edges sharp in the left image we
would have needed to use a very small aperture. The central image, however,
is able to achieve this even with the same aperture. On the other hand, note
how the vertical depth of field has decreased and caused the top of the front
lens to be blurred.

Tilt can also be used to reduce apparent depth of field, as demonstrated
by the 8° upward tilt image. This can be particularly useful for portraits
when a wide aperture is insufficient, or when one wishes to focus on
only part of a vertical object. Note how both the rug and vertical depth
of field appear to have decreased. This is because the focus plane is
at an angle in between the rug and lens. Also note how the field of
view has moved downward due to the tilt, which should be taken into
account.

Another possibility would be to place the depth of field both above
and parallel to the rug, such that only the tops of the two lenses are
in sharp focus (right image). This type of placement is common for many
types of flower shots, since these have a geometry similar to this rug/lens
example.

For landscapes and architecture, however, the goal is usually to
achieve maximal sharpness throughout. In the rug/lens example this would
require placing the focus plane slightly above and parallel to the rug
with a small aperture.

Deciding where to optimally place the focus plane can become a tricky
game of geometry, particularly if the subject traverses both front/back
and up/down directions. This requires considering not just the angle
of the focus plane, but also the shape of the depth of field.

Downward Tilt
Only Lens Tops in Focus

mouseover to view at f/5.6

Instead of the usual rectangular region for an ordinary lens, the depth of
field for a tilt shift lens actually occupies a wedge which broadens away from
the camera. This means that placement of the depth of field is more critical
near foreground subject matter.

Large Aperture

Small Aperture

Large Aperture

Small Aperture

Ordinary Camera Lens

Tilt Shift Lens

Blue intensity qualitatively represents the degree of image
sharpness at a given distance;
actual depth of field can be unequally distributed to either side of the
focus plane.

Note how using a small aperture with a tilt shift lens can become very important
with vertical subject matter—and increasingly so if this subject matter is in
the foreground, or with a more horizontal focus plane.

Traditional view cameras (ie, old-fashioned looking camera with flexible
bellows) can use virtually any amount of lens tilt. However, the Nikon and Canon
tilt shift lenses are limited to 8.5 and 8 degrees of tilt, respectively. This
means that achieving optimal sharpness throughout is often a compromise between
the best possible location for the focus plane and the constraints caused by
a narrow range of tilt. This can sometimes occur when one requires a horizontal
focus plane, since this may not always be achievable with just 8 degrees of
tilt. The example below demonstrates an alternative placement:

The key is to optimally place not only the focus plane, but also its wedge-shaped
depth of field. Note how in the right image the focus plane crosses the
floor, which ensures that depth of field is most efficiently distributed across
the floor and two subjects. In this example the crossover distance is positioned
just before the
hyperfocal distance of the corresponding untilted lens, since there is minimal
vertical subject matter. For other subject distributions, proper placement depends
on the relative importance of subject matter and the artistic intent of the
photograph.

A more sophisticated possibility is to use a combination of tilt and shift.
This could have been accomplished by first aiming the camera itself slightly
towards the ground, thereby rotating the focus plane even further than possible
using lens tilt alone. One could then use
shift to change the field of view—thereby maintaining a similar composition
as the original unshifted camera angle, but with a different perspective.

Overall, even if one cannot tilt enough to place the focus plane at the best
possible location, one can usually still use some tilt and be better off than
what would have been achievable with an ordinary lens. The only exception is
when there is vertical subject matter in the foreground which fills a significant
fraction of the image, in which case zero tilt is usually best, although shift
movements are likely to be helpful.

FOCUSING A TILT SHIFT LENS

Mentally visualizing how tilting a lens will correspond to changes in the
depth of field can be quite difficult, even for the most experienced of photographers.
Even then, knowing where to best place the focus plane is only half the battle—actually
putting it there can be a different matter entirely.

The reason focusing can become so difficult is because the focusing distance
and the amount of tilt do not independently control the focus plane's location.
In other words, changing the focusing distance changes the angle of the focus
plane in addition to changing its distance. Focusing can therefore become an
iterative process of alternately adjusting the focusing distance and lens tilt
until the photo looks best.

Perhaps the easiest scenarios are those which demand more tilt than the lens
supports. In these cases one can just use maximal tilt in the chosen direction,
then choose the focus distance which achieves the best available depth of field
placement. No tilt/focusing iterations are required.

For more difficult focusing scenarios, tilt shift lenses are usually focused
using trial and error techniques through the viewfinder. This works by following
a systematic procedure of alternating between setting the focusing distance
and tilt, with the aim of having the focus plane converge onto the desired location.
Since accurate focusing requires consistent and careful attention to detail,
using a tripod is almost always a must.

The following procedure is intended for situations where the subject lies
primarily along a horizontal plane or some other plane which is rotated relative
to the camera's sensor:

Focusing Procedure for a Tilt Shift Lens

(1) Compose

Set lens to zero degrees tilt and frame the photograph

(2) Identify

Identify critical nearest and furthest subjects along the subject
plane

(3) Focus

Focus at a distance which maximizes near and far subject sharpness
in the viewfinder (if far subject is at infinity, this distance will
be at or near the hyperfocal distance).

Once an approximate distance is identified, rock the focus ring back
and forth slightly to get a better estimate of this distance.

(4) Tilt

Very slowly apply progressively more lens tilt towards the subject
plane until near and far subject sharpness is maximized in viewfinder.

Once an approximate tilt angle is identified, slightly rotate the
tilt knob back and forth to get a better estimate of this angle.

(5) Refine

Repeat steps (3) and (4) with smaller changes than before to identify
whether this improves both near and far subject sharpness; if no further
improvement then the focusing procedure is complete.

For more on step (3) above, see tutorials on
depth of field and the
hyperfocal distance.
For landscapes, one should generally put more weight on having the furthest
subject sharp.
Overall, the above procedure aims to give robust results across a wide range
of scenarios;
for more exact focusing under specific conditions, refer to the calculators/charts
later in this tutorial.

Also note that using the camera's focus points and focus lock confirmation
can be of great help. Even though tilt shift lenses do no work with a
camera's autofocus, your camera can still be used to notify when you have
achieved successful manual focus. Select a focus point which is on your subject
and use the focus lock lights in the viewfinder to confirm when your tilt or
focusing has successfully brought this subject into focus.

With practice, visual procedures work fine, but ultimately nothing beats
having a better intuition for how the process works. One is encouraged to first
experiment heavily with their tilt shift lens in order to get a better feel
for using tilt movements.

TOOLS TO MAKE TILT SHIFT FOCUSING EASIER

Trial and error techniques can be problematic due to the limited size of
viewfinders used in 35 mm or cropped digital camera sensors. This can make it
very hard to discern changes in sharpness—particularly in low light, or with
tilt shift lenses having a maximum aperture of f/3.5 or a wide angle of view.
However, several tools are available which may make this process easier.

A special texturized manual focusing screen can ensure the eye has
a clear reference to compare with out of focus objects. Otherwise one's eye
can get tricked and try to make objects appear in focus even though these objects
are not necessarily in focus in the viewfinder. In such cased the eye effectively
becomes a part of the optical system.

Alternatively, if your camera supports real-time viewing using your camera's
LCD (Live View) this can be of great help. One can also take a series
of test photographs and then zoom in afterwards to verify sharpness at critical
points.

A magnified viewfinder can also help, such as Canon's "angle finder
C" or one of many third party viewfinder magnifiers. Many of these are at a
right angle to the viewfinder, which can make for more convenient focusing when
the camera is near the ground.

FOCUSING TECHNIQUE FOR LANDSCAPE PHOTOGRAPHY

Tilt movements for landscape photography often require a focus plane which
lies along a sweeping, near horizontal subject plane. In these situations it
is very important to place the focus plane accurately in the foreground. The
vertical distance "J" is easy to set because it is only determined by the lens
tilt, not focusing distance.

Once the desired value of J has been determined and the corresponding tilt
set, one can then independently use the focusing distance to set the angle of
the focus plane. Setting the lens's focus ring to further distances will simultaneously
increase the angle of the focus plane and the angular depth of field, as demonstrated
in the next section.

TILT SHIFT LENS DEPTH OF FIELD CALCULATOR

The calculator below uses the lens focal length, lens tilt and untilted focusing
distance to locate the plane of sharpest focus and depth of field:

Tilt Shift Lens Depth of Field Calculator

Camera
Sensor Size

Aperture
and sensor size are only necessary if you also wish to estimate
depth of field:

Aperture

Focal Length of Tilt/Shift Lens

mm

Tilt Amount

degrees

Untilted Focus Distance

meters

Angle of
Plane of Sharpest Focus (Focus Plane)
Vertical Distance
of Focus Plane from Lens (J)Angle of Near Plane of Acceptable SharpnessAngle of Far Plane of Acceptable SharpnessTotal Angular Depth of Field

Calculator is a work in progress, but should give adequate
estimates for most scenarios.
Assumes thin lens approximation, with greatest error for focusing distances
near infinity or close up.
The circle of confusion is the standard size also used in the regular
depth of field calculator.
"Untilted Focus Distance" is (approximately) the distance labeled on
your lens's focus ring.

Note how small changes in tilt lead to large changes in the focus plane angle,
and that tilt is correspondingly less influential as the tilt angle increases.
Also observe how the untilted focusing distance can have a significant impact
on the focus plane angle. Similar to ordinary depth of field, the total angular
depth of field (near minus far angles of acceptable sharpness) decreases for
closer focusing distances.

USING SHIFT TO FURTHER ROTATE THE FOCUS PLANE

The next calculator is useful for situations where tilt and shift are used
together to achieve an even greater rotation in the focus plane. With an ordinary
camera lens, the angle of the focus plane changes when the camera is rotated,
since the focus plane is always perpendicular to the lens's line of sight. With
a tilt shift lens this is no different. However, the key is that with a tilt
shift lens, we can rotate the camera slightly and then use a shift movement
to ensure the same field of view (same composition).

Ordinary Unrotated Lens

Rotated Ordinary Lens
Rotates plane of focus,
different field of view

Rotated Lens With Shift
Rotates plane of focus,
maintains field of view

Blue intensity qualitatively represents the degree of image
sharpness at a given distance;
light gray line lies along the center of the photograph.

The calculator below demonstrates how much one would have to rotate their
camera in order to offset a given lens shift, which is also equal to the rotation
in the focus plane. This would achieve a rotation in the focus plane similar
to the top left and right images above, with also the same field of view.

Using Shift to Rotate the Focus Plane

Focal Length of Tilt/Shift
Lens

mm

Shift Amount

mm

Amount
of Focus Plane Rotation

Rotation in focus plane is relative to its location
for camera/lens with same field of view, but no shift.

Note how shift can rotate the plane of sharpest focus much more for shorter
focal lengths. This is because in absolute units, a given mm shift corresponds
to a greater rotation in in the field of view. On the other hand, this also
means that the perspective will be more strongly influenced for shorter focal
lengths, which may be an important consideration.

Be aware that using shift to rotate the focus plane may require that your
tilt shift lens be modified so that it can tilt and shift in the same direction,
which is usually not the case by manufacturer default. This can be sent to the
manufacturer for modification, or can be performed yourself with a small screwdriver.
One needs to remove the four small screws at the base of the lens, rotate the
base 90°, and then screw them back into the base.

AVAILABLE NIKON & CANON TILT SHIFT LENSES

Canon has four and Nikon has three mainstream tilt shift lens models available:

Canon Tilt Shift Lenses

Nikon Tilt Shift Lenses

Canon 17 mm TS-E f/4L

Canon 24 mm TS-E f/3.5L II

PC-E Nikkor 24 mm F3.5D ED

Canon 45 mm TS-E f/2.8

PC-E Nikkor 45 mm F2.8D ED

Canon 90 mm TS-E f/2.8

PC-E Nikkor 85 mm F2.8D ED

Calculations and diagrams above have been designed to represent the range
of tilt and shift movements relevant for these lenses on the 35 mm and cropped
camera formats.